In what atom/ion does a 3p electron experience the greatest effective nuclear charge?

The magnitude of the nuclear charge is equal to atomic number of the element.

The 1st and 2nd shells are the inner shells of a 3p electron.
Thus, total no. of electrons in the inner shells = 2 + 8 = 10

Effective nuclear charge = (Nuclear charge) - (Total no. of electrons in the inner shells)
Effective nuclear charge = (Atomic number) - 10
Obviously, the higher the atomic number, the greater the effective nuclear charge is experienced by the 3p electron.

Among the 5 options, Ca²⁺ has the greatest atomic number. ...... Ans: (C)